This invention relates generally to power supplies and, more particularly, to switching power supplies including snubber circuits.
As is known in the art, switching power supplies are frequently used to convert an input voltage from an unregulated source into a regulated voltage at an output thereof. In one such power supply, there is produced a feedback control signal related to the difference between the actual voltage at the output and the desired output voltage. The power supply includes a generator for producing a signal comprising a train of pulses, the width to the pulses being controlled or modulated in accordance with the control signal. A switching transistor responsive to the pulses couples the unregulated input voltage source to and from a load device via an inductor for a time duration in accordance with each pulse width.
During the portion of each period of the pulse train when the switching transistor is enabling current from the input voltage source, that current is used to store energy in the series-connected inductor and to charge a parallel-connected capacitor, as well as supplying the load device. During the remaining portion of each period, i.e., while the switching transistor decouples the supply voltage from the inductor, the polarity across the inductor reverses because of the collapsing magnetic field, and the inductor begins to supply both the load current and the charging current to the capacitor. As the energy store in the inductor discharges, its current falls off and the capacitor begins to supply current to the load. The switched voltage at the transistor side of the inductor is smoothed by the filter comprising the inductor and capacitor and, in the steady-state, this switched voltage becomes the desired output voltage.
With such arrangement, as the switching transistor decouples the input voltage source from the inductor, the voltage across the switching transistor rises rapidly as the polarity across the inductor reverses; however, the current flowing through the switching transistor does not immediately fall to zero, due to a delay (turn-off time) inherent to the switching transistor. The product of voltage across the switching transistor and the current flowing through it during the turn-off time may translate to excessive power dissipation (heat) by the switching transistor. Therefore, for a switching power supply of this type, the power handling capacity of the switching transistor must be increased.
One technique sometimes used to reduce the power handling requirement of the switching transistor is to provide a so-called "snubber circuit." An exemplary dissipative snubber circuit design is disclosed in "High-Frequency Switching Power Supplies: Theory and Design", by G. Chryssis, 1984, pp. 60-63. In this example, the snubber circuit is disposed across the switching transistor and provides a path for the current normally passing through the switching transistor and dissipates the stored energy in a passive (i.e., resistive) load when the switching transistor is conducting. Thus, the snubber circuit comprises a series-coupled resistor and capactior, with the capacitor storing energy during the transistor turn-off period and the resistor dissipating the stored energy as heat. This circuit reduces the power handling requirement of the switching transistor; however, the power conversion efficiency of the power supply is slightly degraded.
An alternative approach to reducing the power-handling capability of the switching transistor is to reduce the turn-off delay of the switching transistor. This is sometimes accomplished by biasing the control electrode of the switching transistor with a voltage sufficient to ensure rapid cut-off, e.g., a negative voltage applied to the gate electrode of an N-channel field effect transistor (FET). However, this approach requires a separate power source to provide the negative voltage.
A non-dissipative snubber disclosed in "Design of Solid-State Power Supplies", by E. R. Hnatek, 1981, pp. 290-292, more particularly, FIG. 7-29, allows energy stored in the snubber to be utilized by the switching power supply instead of being dissipated as heat. However, only a single positive output voltage is available for use and an additional supply would still be needed to provide a negative voltage to ensure cut-off. Additionally, such switching power supplies generally require a lare pulse transformer for driving the switching transistor. The pulse transformer isolates control circuits (e.g., the pulse generator) of the power supply from the switching transistor. Inefficiencies of transformer coupling necessitate use of a high-power driver. Furthermore, a large pulse transformer is required to ensure that the core of that transformer does not saturate during a pulse. Since the high-power driver does not contribute to power supplied by the switching power supply, the overall efficiency of the power supply is thereby reduced. Additionally, the high-power driver and pulse transformer increase the size, weight and cost of the power supply.